Rare earth doped waveguide amplifiers, such as erbium doped fiber amplifiers (EDFAs), are important components in optical communications systems. Such amplifiers provide compact means of amplifying optical signals transmitted on waveguides.
A typical rare earth doped waveguide amplifier comprises a length of optical waveguide typically including a central core and a surrounding cladding. A length of the core is doped with atoms of one or more rare earth elements, and the doped length is exposed to pumping light to excite the rare earth dopants. When signal light passes through the core, the signal is amplified by stimulated emission.
A difficulty with conventional rare earth doped amplifiers is that the gain they provide is a function of signal wavelength, and this spectral dependence is, in turn, a function of temperature. In early applications involving only one or a few closely spaced signal channels, these dependencies were of little consequence.
As systems evolved to several channels, the spectral dependence became a matter of concern, and filters were designed to compensate for it. One approach using long period gratings is described in U.S. Pat. No. 5,430,817 issued to A. M. Vengsarkar on Jul. 4, 1995. Another approach using waveguide coupler filters is described in U.S. Pat. No. 5,578,106 issued to C. H. Henry et al. on Jan. 21, 1997. Both of these patents are incorporated herein by reference. Such filters were adequate to compensate a constant-temperature amplifier over a narrow wavelength range.
As contemplated systems have further evolved to many channels over an even wider bandwidth, the temperature dependence of rare earth doped amplifiers has assumed increasing importance. Moreover the filter arrangements used to compensate the spectral dependence of these amplifiers also are temperature dependent, producing complex problems for advanced systems. The problems presented by the widely used erbium doped fiber amplifiers (EDFAs) are instructive.
In the past erbium-doped fiber amplifiers (EDFAs) were rarely filtered to produce a flat gain spectrum since they were naturally flat enough for use in applications with only a few channels. However, systems have been recently announced that include up to 100 signal wavelengths spaced at 50-100 GHz. Such systems require EDFAs with essentially flat gain spectra spanning 20, 40 or even 80 nm. Typically, gain flatness to within 1 dB is desired to minimize accumulated gain excursion between channels. Such flatness at constant temperature can, with challenging effort, be achieved by complementary loss filters [A. M. Vengsarkar et al., Opt. Lett. 21, 336 (1995)].
Many communication systems require that the EDFAs operate over a wide temperature range depending upon the external environment, heat generated inside the EDFA package and heat generated in the proximity of the package. While the gain of erbium in a silicate host has a low temperature dependence, the EDFA gain spectrum changes enough over typical operating temperature ranges to make maintaining flatness within 1 dB very challenging.
The graphical illustration of FIG. 1 shows this problem. Curve 1 is the ideal flat gain spectrum between 1528 and 1563 nm of an EDFA with 20 dB of gain flattened by a hypothetical ideal loss filter. The hypothetical ideal filter is assumed to achieve flatness at 20.degree. C. (room temperature) and is assumed to be constant with temperature. But even with this ideal filter, changes in temperature produce nonuniformities in the gain spectrum. Curves 2, 3 and 4 show the calculated variation induced by temperatures of 0.degree. C., -20.degree. C. and -40.degree. C., respectfully. Curves 5, 6 and 7 show the variation at 40.degree. C., 60.degree. C. and 80.degree. C. (These variations were calculated using the OASIX EDFA Simulation Tool available from Lucent Specialty Fiber Devices.) As can be seen, even an ideal filter could not flatten the gain curve of the amplifier to within one 1 dB over these ranges of wavelength and temperature. Moreover a real filter, rather than an ideal one, would be challenged to flatten to 1 dB even at constant temperature.
One potential solution to the temperature dependence of EDFA gain is to operate the EDFA in a temperature-controlled environment. However, maintaining temperature near the middle of the temperature range (for example 30.degree. C.) is a costly and energy intensive because it requires either the use of both heating and cooling elements or the use of an energy-inefficient thermoelectric cooler (TEC). The easiest solution is to operate the entire device at a constant hot temperature (for example 60.degree. C.) since heaters are cheap and reliable. But hot temperatures create further complications. Many devices such as pump diodes and some passive components are not designed to operate at high temperatures for prolonged periods. Furthermore, most aging processes are accelerated at high temperature. In particular, the loss in most EDFA amplifiers increases as hydrogen diffuses in and reacts with host materials. This process is dramatically accelerated at high temperatures. In short, temperature controlled EDFA packaging can be expensive to implement, energy intensive to operate and detrimental to the performance of the components in the EDFA.
Accordingly there is a need for an improved temperature-compensated optical waveguide amplifier.